In recent years, arc tubes each of which is capable of realizing a high brightness have widely been used as light sources, for example, the headlights of vehicles.
In general, an arc tube serving as a light source, such as a headlight of a vehicle, as shown in FIG. 5, incorporates a quartz glass arctube body 104 having pinch seal portions 104b formed on the two sides of a light-emission tube 104a constituting a discharge space 102. Moreover, the arc tube incorporates a pair of tungsten electrodes 106 pinch-sealed to the pinch seal portions 104b such that the leading ends of the tungsten electrodes 106 project into the discharge space 102.
The arc tube having the above-mentioned structure is arranged such that each of the tungsten electrodes 106 is electrically polished to smooth the surface of each of the tungsten electrodes 106 to obtain a predetermined discharge characteristic.
From a viewpoint of preventing occurrence of a leak from the arc-tube body 104, experiments conducted by the inventors of the present invention resulted in the following fact. That is, the simple electrolytic polishing process which is performed in order to maintain the discharge characteristic cannot attain a satisfactory smoothness of the surface of each of the tungsten electrodes 106.
When the surface of each of the tungsten electrodes 106 has some roughness, the tungsten electrodes 106 and the pinch seal portions 104b are engaged to each other with great pits and projections, as shown in FIG. 6, after the tungsten electrodes 106 have been pinch-sealed to the pinch seal portions 104b. Therefore, excessively large compressive stress is maintained in a region adjacent to the surfaces of the pinch seal portions 104b with which the pinch seal portions 104b are joined to the tungsten electrodes 106. The large compressive stress causes a crack of the arc-tube body 104 to occur during use of the arc tube. Thus, a leak occurs been the discharge space 102 and the extena space. Therefore there arises a problem in that the life of the conventional arc tube is unsatisfactorily short.
In view of the foregoing, an object of the present invention is to provide an arc tube which is capable of preventing occurrence of a leak caused from a crack of the arc-tube body so as to prolong the life of the arc tube, and a manufacturing method therefor.